The present invention relates generally to the field of motor boats and particularly to those intended to be operated at high speed.
A motor boat which is intended to be operated at high speed will generally be of the planing type and should be so designed as to have low resistance in the high speed planing condition. When the resistance is low, then the desirable features of low values of engine power, boat cost, and fuel consumption rate are achieved. An important additional benefit is conferred when a planing boat intended for towing water skiers has low resistance at high speed. For this application a minimum of turbulence in the wake behind the boat is desirable. With reduced resistance,the propulsive effort required from the propeller is lessened, and the turbulence in the wake behind the boat, resulting from the propeller action, will be beneficially diminished.
Now, it should be stressed that in a representative case of a boat planing at a speed of 45 mph, the primary source of resistance is the frictional drag on the hull bottom as the boat slides over the surface of the water. Typically, the frictional resistance will then account for about 85% of the hydrodynamic hull drag. Furthermore, the frictional resistance is directly proportional to the magnitude of the hull bottom area that is wetted in the planing condition. Again, typically, at a speed of 45 mph each square foot of wetted bottom area produces nearly 10 pounds of resistance; and at 55 mph each square foot of wetted bottom area produces nearly 15 pounds of resistance.
In the past, designers of planing motor boats have tried various configurations of steps in the hull bottoms, in an attempt to separate the flow from portions of the bottoms, and thereby achieve a reduction in the extent of wetted bottom area, and consequently in resistance. For a variety of reasons these solutions have been unsatisafctory, and therefore practically all fast planing boats are of the unstepped type--despite the fact that their large magnitudes of bottom wetted area result in high values of resistance at high speeds.
One of the unsuccessful practices of the past has been to introduce an essentially transverse step in such a way that all of the hull bottom behind the step is clear of the water surface at high speed. This configuration is unsatisfactory because reducing the wetted area in this manner also results in a shortening of the planing surface, which tends to cause porpoising. Also, stepped bottom configurations in which the aftermost part of the hull bottom behind the step is in contact with the water surface were found wanting because no single position of this rear surface is suitable for the different speeds, loads, and water surface conditions at which a boat operates. This finding led to the introduction of rear planing surfaces which could be adjusted to suit the various conditions; however, this solution has the drawbacks of increased cost and complexity, and therefore has failed to find acceptance and utilization.
In addition to the drawbacks mentioned above, the various stepped boat designs of the past are found to have further shortcomings when applied to inboard-powerd boats for towing water skiers. For this application, a relatively smooth, turbulence-free wake is a primary desideratum. One of the necessary preconditions for this result is for that portion of the hull bottom which is above the shaft, strut, propeller, and rudder, to remain in contact with the water surface. This requirement is not satisfied by the various stepped designs of the past, because they leave one or more of the appendages (shaft, strut, or rudder), uncovered and piercing the free water surface. In this condition they produce undesirable amounts of spray and turbulence in the wake behind the boat. Furthermore, the propeller will lose effectiveness if it is not covered by a region of hull bottom which is in contact with the water surface, since if it is uncovered it will tend to draw ventilating air down from the free surface. In this condition the propeller will also produce an undesirable air-filled wake. The shaft, strut, and rudder will also tend to draw air down from a free water surface, particularly when the boat is turning. In the case of the strut, this will aggravate ventilation of the propeller, and in the case of the rudder, both its effectiveness in providing directional stability and in providing a side force during turns will be impaired.
Another unsatisfactory and unsuccessful past practice has been to introduce into the bottom of a planing boat a transverse step together with a pair of inwardly converging diagonal side steps. This configuration, which is shown in U.S. Pat. No. 3,149,351, is unsatisfactory in several respects. First, because it shortens the planing surface it causes porpoising. Second, the straight diagonal side steps depicted in the referenced patent form an angle of approximately 160 degrees at their junctures with the chines. Now, at intermediate speeds water flowing along the sides of the hull will not separate at such junctures, but will instead adhere to them by suction, and will therefore continue to wet the faces of these side steps and also the bottom regions behind these steps. Accordingly, the desired flow separation from the bottom regions behind the steps will not be achieved. A further drawback is that the straight diagonal steps depicted in the referenced patent meet the chines at 32 percent of the hull bottom length forward of the stern. This intersection point is much too far forward. It will result in the bottom edge of each step being crossed by the line of the forward boundary of the wetted region of the bottom of the boat when it is planing at high speed. In this situation the flow will reattach to the hull bottom behind each step, and therefore the desired reduction in hull bottom wetted area will not be achieved. The line of the forward boundary of the wetted bottom region as shown in the referenced patent is incorrect. It is shown making an angle of 40 degrees with the centerline of the boat (in plan view). In actuatlity this line will typically make an angle of less than 20 degrees with the hull centerline, and this will result in its crossing the diagonal side steps which are shown and described in the referenced patent.
The transverse step of the foregoing configuration has the further drawback that, if utilized for an inboard-powered boat, it will leave the rudder and propeller without a covering by wetted hull bottom area. In this condition they will lose effectiveness, and also produce excessive spray and turbulence in the wake behind the boat. The sharp corners at the junctures between the diagonal side steps and the transverse step, which are indicated in the referenced patent, also have the drawback that each of these corners will produce a ridge of water in the wake behind the boat, thereby disturbing the desired smooth wake.